1. Field of the Invention
This invention relates to an apparatus for effectively erasing the noise developing in a visible image reproduced from a stimulable phosphor sheet carrying a radiation image. This invention particularly relates to an apparatus for effectively erasing the noise developing in such a reproduced visible image due to repeated use of the stimulable phosphor sheet in a radiation image information recording and reproducing method where the stimulable phosphor sheet is exposed to a radiation to record a radiation image therein and then exposed to a stimulating ray to emit light in the pattern of the stored image, the emitted light is converted to an electric signal, and a visible image corresponding to the radiation image is reproduced by use of the electric signal.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to such radiation as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays or ultraviolet rays, they store a part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to a stimulating ray such as a visible ray, light is emitted from the phosphor in the pattern of the stored energy of the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. Nos. 4,258,264, 4,276,473 and 4,340,911, U.S. patent application Ser. No. 220,780 (now U.S. Pat. No. 4,387,428) and Japanese unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use the stimulable phosphor for recording a radiation image of an object. Specifically, the stimulable phosphor is first exposed to a radiation to have a radiation image stored therein, and is then scanned with a stimulating ray which causes it to emit light in the pattern of the stored image. The light emitted from the stimulable phosphor upon stimulation thereof is photoelectrically detected and converted to an electric image signal, which is processed as desired to reproduce a visible image of a quality suitable for viewing and diagnostic purposes.
In the radiation image recording and reproducing method described above, the final visible image may be reproduced in the form of a hard copy or may be displayed on a cathoderay tube. The stimulable phosphor sheet used in this method may be in any of various forms such as a panel, drum or the like, which are herein generally referred to as sheets. For economical reasons, it is desirable that the stimulable phosphor sheet be used repeatedly in many separate radiographic operations.
In order to reuse the stimulable phosphor sheet, it is necessary that the stimulable phosphor sheet to be reused be made completely free from the previously stored radiation image. Theoretically, the radiation energy of the radiation image stored in the stimulable phosphor sheet should disappear when the sheet is scanned with a stimulating ray of a sufficient intensity to cause light to emit therefrom in the pattern of the stored radiation image in the course of the radiation image recording and reproducing process as described above. Actually, however, the stored radiation energy cannot be completely eliminated only with the stimulating ray used to scan the stimulable phosphor during the aforesaid process. Thus a part of the previously stored radiation image remains in the reused stimulable phosphor sheet and inconveniently causes noise to occur in the visible image reproduced from the reused stimulable phosphor sheet. In order to successfully reuse the stimulable phosphor sheet, any such residual radiation image thereon must be erased completely before reuse.
Further, a stimulable phosphor contains a little amount of radioactive isotopes such as .sup.226 Ra and .sup.40 K, which emit radiations and cause the stimulable phosphor sheet to store the emitted radiation energy even when the sheet is not being used in exposing to the imagewise radiation. These traces of radioactive isotopes also constitute a cause of the noise developing in the reproduced visible radiation image. Furthermore, a stimulable phosphor sheet is also affected by environmental radiations such as cosmic rays and X-rays emitted from other X-ray sources and stores the energy thereof. These types of radiation energy (hereinafter referred to as fog) undesirably stored in the stimulable phosphor sheet also cause noise to appear in the visible radiation image reproduced from a reused stimulable phosphor sheet and, therefore, must be erased before reusing the stimulable phosphor sheet.
Now, it has been found that, in order to eliminate the detrimental noise due to the residual radiation image, the radiation energy of the radiation image stored in the phosphor must be erased to the order of 10.sup.-4 to 10.sup.-6. Stated differently, the original radiation energy stored in the phosphor must be erased to a level between 0.01 and 0.0001 when the maximum level of the stored radiation energy at the recording of radiation image information is 100. On the other hand, the level of the fog developing in the phosphor is generally about 0.1 to 0.001 based on the maximum of the stored original radiation energy which is taken as 100 as described above. It has also been found that the fog must be erased to a level between about 0.01 and 0.0001 in order to prevent the fog from developing detrimental noise in the next radiographic operation.
Under the above circumstances, in Japanese patent application Nos. 56(1981)-2970 (corresponding to U.S. Pat. No. 4,439,682) and 56(1981)-4889 (corresponding to U.S. Pat. No. 4,438,333) which are applied by same assignee of this invention a noise erasing method and apparatus which comprises two erasing steps wherein the first erasing which requires a long period of time to erase the previously stored image is carried out at an appropriate point of time after the radiation image is stored in a stimulable phosphor and scanned with a stimulating ray, and the second erasing for the fog which can be erased quickly is conduced immediately before the next imagewise exposing to radiation.
However, the above-mentioned system requires two erasing operations using two kinds of separate apparatus for the first and second erasing operations, and therefore is expensive and troublesome.